Why Is an IGBT Better than a BJT or MOSFET ??

Nowadays, in industries there is a need of converters used at vast voltage and power range (fractional kW to several MW at frequency less than 10kHz). However, the traditional power Bipolar Junction Transistors or the power Metal Oxide Semiconductor Field Effect Transistors cannot do the job very well.

Both the BJT’s and MOSFET’s have their own Pros and Cons

Bipolar Junction Transistor

Pros:

1. Lower ON State Conduction Losses

(Better Efficiency when ON)

2. Handle higher voltages

Cons:

1. Longer switching rates since Bipolar

2. Threat of secondary breakdown due to generation of local hotspots

Metal Oxide Semiconductor Field Effect Transistor

Pros:

1. lower Switching rates since unipolar

(can be switched on and off faster)

Cons:

1. Higher On state losses (reduced efficiency when ON)

2. Cannot handle higher voltages

However, we would obviously love a device that has advantages of both the above mentioned devices. And hence, the Insulated Gate Bipolar Transistor (IGBT) was born. A 3 terminal device that combines BJT’s Low ON-State conduction losses even at high Voltages with MOSFET’s high switching speeds (up to 30kHz).

The 3 terminals of an IGBT are

G => Gate, C => Collector, E => Emitter.

The labels were stolen from the BJT/ MOSFET. Gate comes from the MOSFET and Collector, Emitter come from the BJT.

BJT + MOSFET = IGBT

Like the BJT/ MOSFET, IGBT also has 2 types

1. NPN; 2. PNP

However, since the PNP IGBT delivers inferior characteristics, it is hardly used.

If we look at the equivalent circuit of an IGBT, we can see that it consists of N-channel MOSFET and a P-channel BJT.

Thus, N-channel MOSFET contributes to the high input impedance of IGBT and the P-channel BJT contributes to the low output impedance of IGBT.

Even though IGBT has many advantages, it also has 1 major disadvantage:

Latching Problem:

This phenomenon is observed when Gate terminal loses control over the Collector Current. Because of which, even though this Voltage controlled device has a voltage lower than Gate-Emitter Threshold voltage, IGBT does not shut down. Thereby, an external commutation circuitry is required to turn OFF the IGBT which increases complexity when designing a circuit including IGBT.

That's all for this post! Hope you now have an idea of what an IGBT is and how it compares to an BJT or a MOSFET.